1. Field of the Invention
This invention relates to an image forming apparatus and more specifically to the image forming apparatus including a curl correcting unit configured to correct a curl caused in a sheet.
2. Description of the Related Art
Hitherto, an image forming apparatus such as a copier and a printer is configured to transfer a toner image formed in an image forming unit thereof on a sheet fed from a sheet feeding portion and then to guide the sheet to a fixing unit to fix the toner image on the sheet. Among such fixing units, there is a thermal pressure fixing-type fixing unit configured to fix the toner image on the sheet by pressurizing and heating the sheet passing through the fixing unit. Still further, among the conventional image forming apparatuses, there is one that is provided with a sheet processing unit to be able to readily carry out such processes as sorting, stapling, punching and book-binding of the sheets.
By the way, the sheet on which the toner image has been fixed in the fixing unit is liable to cause a curl by absorbing moisture condensed on a guide of the fixing unit and vapor in a surrounding area. If the sheet thus curls, there is a possibility of not only causing jamming of the sheet but also of dropping performance in stacking the sheet on a tray after its discharge. Then, in order to correct such a curl of the sheet, there is one provided with a curl correcting unit between the image forming apparatus and the sheet processing unit.
Among such curl correcting units, there is a roller nip-type curl correcting unit composed of an elastic roller and a pressure roller that rotates while forming a curved nip portion by pressurizing the elastic roller. The curl correcting unit is configured to correct (to straighten) the sheet so as to have no curl by temporarily curving and deforming the sheet in a direction opposite from a direction in which the sheet is curled.
It is noted that condition of the curl of the sheet differs depending on density of the image formed on the sheet and on a difference of environmental humidity. Therefore, the curl correcting unit is configured to able to adequately correct the curl by adjusting a force of the pressure roller, i.e., an amount of bite of the pressure roller to the elastic roller, corresponding to such detected information.
Still further, among the late image forming apparatuses, there is one that is configured to increase sheet conveying speed in passing the sheet through the curl correcting unit to improve productivity of the apparatus. However, if the sheet conveying speed increases as described above, there is a possibility that the curl is not fully corrected because a time during which the sheet passes through the curl correcting unit is shortened in this case. Then, in order to prevent the curl from being defectively corrected in the image forming apparatus as described above, Japanese Patent Application Laid-open No. 2007-161398 discloses an arrangement of increasing the pressurizing force in connection with the increase of the sheet conveying speed.
In order to accommodate to a variety of needs of users, some image forming apparatuses include a consolidated mode of forming images on different types of sheets and of conveying the sheets to the sheet processing unit in one JOB. However, because a degree of curl of the sheet is different depending on the types of sheets, it is necessary to switch the amount of bite of the pressure roller to the elastic roller in such consolidated mode corresponding to the respective types of sheets.
However, with the increase of the sheet conveying speed, an interval or a distance between a rear edge of a preceding sheet and a front edge of a succeeding sheet tends to be narrowed in the late image forming apparatuses, so that it is difficult to complete the operation for switching the amount of bite temporally during the interval between the sheets described above. Then, there is a possibility that the correction of curl cannot be fully made on the different types of sheets when it is unable to complete the operation of switching the amount of bite during the temporal interval between the sheets.